This audio speech compressor is designed around the Analog Devices SSM2165 device. This IC gives a 40 dB compression range and interfaces nicely with typical electret microphone elements. I decided to build this up because the FT-817 doesn't have any type of speech processing built-in, and any extra oomph helps when you are QRP.

The circuit is small enough that it can be built into the microphone. However, I decided to go external until my FT-817 is out of warranty. I've built up a unit using the SMD version of the SSM2165 for this.

I purchased the SSM2165 from Newark Electronics ($4.25). You'll need a pc-mount RJ45 jack and some perf-board. For the cable to connect to your radio, I just bought an RJ45 cable and cut off one end and wired it to the board. The connectors, cable, and other parts were purchased from All Electronics and Hosfelt Electronics. When everything was finished, I covered the entire assembly with heat-shrink tubing to make it a little more rugged.

So how does it work? According to my Diamond SX-1000 peak/average power meter, the peaks stay the same (as they should), but the average power increases about 6 dB. On the air reports are that the compressor makes my signal sound louder, and I should leave it in all the time.

While I built this compressor for my FT-817, it should work fine with any rig using an electret microphone element. With the component values shown, the unit has unity gain along with 40 dB compression.

I note you didn't add any shielding or intensive RF decoupling. Shouldn't be required for 5 Watts, anyway. However for "others" who may want to add something similar to a higher-powered rig, I'd suggest using a shielded box with ground wires attached to the box, maybe ferrite beads on the audio lines, etc.

Great article! Analog Devices audio I.C.'s sound really good and this project will work as suggested. I built an outboard speech processor using the deluxe version of the same I.C., called the SSM2166. It adds a variable noise gate, a wider VCA gain range (18 dB) and an adjustable rotation or "knee" point. I recommend the SSM2165 for those looking for a simple compressor; the SSM2166 provides performance similar (or equal to) outboard "audio-chain" type compressor units used by the "Hi-Fi" crowd -- but for a lot less $$. Analog Devices' website says both I.C's are currently available in a SOIC (surface-mount) package -- so if Newark Electronics has the standard plastic/epoxy dip versions available they are selling old stock (get 'em while you can). My only complaint building the compressor was the time it took for me to wire-up the surface-mount I.C. Other than that it works fantastic. The noise gate is very smooth in operation -- people won't notice it's there until you turn it off. Manufacturers should build these little gems into their rigs (hint-hint to Scott at Ten-Tec!) For a schematic of the deluxe SSM2166 version, check this website: http://www.muzique.com/ssm2166.htm For the Analog Devices website: http://products.analog.com/products/info.asp?product=SSM2166 73's, Carl

Someone should kit this with a PCB and surface mount components. It would fit very nicely in the mike and is a needed mod for the 817. The only drawback is the battery life may be greatly reduced since you would be running a higher average power than before.

I've noticed some FT-817 rigs having RF-Audio feedback problems when operated on HF /PM (pedestrian mobile) with whip antennas. A speech compressor mounted externally at the mic connector may increase the probability of RF-Audio feedback. Sheilding, suppression, and choke techniques would be beneficial in the design of an in-line mic device. Mounting the compressor inside the microphone case would probably be beneficial as well (shield it in a small can if possible). The stock SSB audio of the FT-817 needs all the help it can get, so BRAVO! on your project.

I agree with Bonnie's comment and would like to
point out that if mounted inside the Microphone
case, the foot print and complexity decrease since
you no longer need the connectors and the shielding
can be made to form fit.

By the way, I am a regular user of the Minibac and
its variations. Thanks Bonnie!

I want to build one! However, i see in the article it mentions that it should work fine with any electret mic, I want to use this for my FT-7 with an Astatic low-z dynamic mic, does anyone see any problems with this? What modifications would you recommend for this?